expect(subject, assertionName, [value, ...])

Perform an assertion about subject.

The expect function will throw an UnexpectedError if the assertion can be decided synchronously and isn't fulfilled:

expect(123, 'to equal', 456);
expected 123 to equal 456

In all other cases expect will return a Bluebird promise which will either be rejected or fulfilled. For consistency, even successful synchronous assertions will return a promise.

The idea is that you can return the promise to your test framework (from an it block or equivalent), so that the outcome of the test will be decided by whether the promise is fulfilled. This works natively in mocha 1.18+, and Unexpected does some unholy trickery so it also works in Jasmine.

Note that if the assertion is asynchronous, you'll have to return the promise to the it block:

it('should call the callback', function () {
    return expect(setImmediate, 'to call the callback');
});

Otherwise your test framework will assume that the test has passed and won't wait for the asynchronous work to complete.

As of 8.0.0 Unexpected will detect created promises that were never returned and make the test fail synchronously. This will uncover some extremely nasty bugs where the test suite succeeds when it should actually fail. However, this feature only works in Mocha and Jasmine.

expect(...).and(assertionName, [value, ...])

The returned promise will be augmented with an and method that allows you to perform more assertions on the same subject:

expect('abc', 'to be a string').and('to have length', 3);

Again, note that you need to return the value returned by expect to your it block if any of the assertions are asynchronous:

it('should do the right thing', function () {
    return expect(setImmediate, 'to be a function').and('to call the callback');
});